The present invention generally relates to the field of satellite based telecommunications and more specifically to a satellite channelizer subsystem for relaying communications signals between earth stations and mobile terminals based on predefined frequency plans.
Satellite based telecommunications systems have been proposed in the past in which a constellation of satellites orbit the earth while relaying bi-directional communications signals between mobile terminals and earth stations. In addition to supporting communication links between mobile terminals, these systems can connect mobile terminals to standard telephone subscribers, through earth station connections to public switched telephone networks.
By way of example only, FIG. 1 generally illustrates a subsection of a proposed system in which a satellite 10 relays radio frequency (RF) signals between mobile terminals 12 and 13 and earth stations 14 and 16. Communications links between mobile terminals 12 and 13 and satellite 10 are generally illustrated by references 18 and 19, while first and second feeder links between the satellite 10 and the first and second earth stations 14 and 16 are generally illustrated by references 20 and 22, respectively. Earth stations connect mobile terminals to fixed subscribers (not shown) via public switched telephone networks. Separate wideband terrestrial networks 11, 17 and 21 (e.g., fiberoptic cable, microwave links, etc.) carry user communications traffic and signaling information between earth stations.
The earth or ground stations 14 and 16 each transmit RF signals over forward links, via the satellite 10, to the mobile terminals 12 and 14. The mobile terminals 12 and 14, in turn, transmit RF signals over return links to the earth stations 14 and 16.
Each satellite 10 includes at least one mobile antenna array (not shown) which defines a mobile coverage area 24 upon the earth surface, and at least one feeder link antenna or array (not shown) which can be pointed independently of the mobile antenna array. Feeder links may be operated at frequencies other than those assigned to the mobile links. The satellite 10 transmits and receives RF signals to and from mobile terminals and earth stations within the coverage area 24. The mobile antenna array divides the coverage area into multiple adjacent mobile link beams 26. Each beam 26 contains multiple frequency subbands having staggered central frequencies. In a single beam, contiguous groups of subbands may be combined to form mobile link channels.
The satellite 10 receives and transmits RF communications signals to and from mobile terminal 12 over one of the mobile link frequency subbands associated with the beam in which the mobile terminal 12 is located. Each mobile link subband may simultaneously carry RF signals to and from multiple mobile terminals at a common center frequency through the use of code division multiple access (CDMA) techniques. Each mobile terminal is assigned a unique CDMA chip code which is embedded within RF signals transmitted to and from the mobile terminal 12. A mobile terminal to satellite bi-directional communications link (having forward and return links) established over a predefined mobile link subband and having a predefined CDMA chip code is referred to as a "resource". Multiple resources are associated with a single mobile link subband. Each mobile link subband may only support a limited number of resources. As noted above, mobile link subbands are processed in groups, referred to as mobile link channels for simplification. Each beam includes at least one mobile link channel. Additional mobile link channels may be assigned to a single beam when the mobile terminal demand within the beam exceeds the number of resources available within a single mobile link channel (e.g., when a beam passes over a major metropolitan area). By way of example only, four mobile link channels may be available for use with a single beam. FIGS. 2 and 3 illustrate exemplary feeder link and mobile link channel definitions which may be utilized in connection with a satellite based telecommunications system. FIG. 2 illustrates the feeder link frequency spectrum 50 (e.g., 300 MHz) associated with a single ground station, which may be divided into a plurality of fixed bandwidth subsections (each of which is referred to as a "channel"). The feeder link frequency spectrum may be divided into a number of channels equaling the number of beams defined by the antenna array so that each ground station can service traffic in any beam. In the example of FIG. 2, 61 beams are utilized and thus the feeder frequency spectrum is divided into 61 channels of equal bandwidth (e.g., 4.9 MHz in width for a feeder link of 300 MHz in width).
FIGS. 3A and 3B illustrate exemplary channel bandwidths which may be utilized for each mobile link beam. In the example of FIG. 3A, the bandwidth available for use within a single beam may be divided into three equal bandwidth mobile link channels, with each channel nominally assigned to one of a plurality of ground stations. Optionally, as in the example of FIG. 3B the mobile link beam bandwidth may equal 16.5 MHz which may be divided into two subbands of 4.9 MHz in bandwidth and two subbands of 3.35 MHz in bandwidth. The configuration of FIG. 3B enables full use of the allocated mobile link band in some beams, when the allocated feeder link bandwidth is not sufficient to support the full mobile link band in all beams simultaneously.
Optionally, each satellite may support multiple feeder links simultaneously and allow multiple earth stations to simultaneously share the bandwidth available to each beam. In the example of FIG. 1, first and second earth or ground stations 14 and 16 communicate over first and second feeder links 20 and 22 with satellite 10. Each of feeder links 20 and 22 may include a feeder bandwidth 50 divided as illustrated in FIG. 2 among a plurality of beams. The mobile link beam bandwidth 60 (FIG. 3B) associated with a single beam 26 may be divided into mobile link channels 62-65. Optionally, each mobile link channel 62-65 may be mapped or assigned to a different feeder link depending upon the number of earth stations in view of, and feeder links supportable by, the satellite 10. If it is assumed that satellite 10 supports three feeder links, then mobile link channel 63 may be assigned to a first feeder link 20 with earth station 14, mobile link channel 64 may be assigned to a second feeder link 22 with earth station 16 and mobile link channels 62 and 65 may be assigned to a third feeder link 23 with earth station 15.
According to the example of FIGS. 1-3, earth station 14 may transmit RF signals over a desired subband of first channel 52 of feeder link 20. The satellite 10 translates in frequency the RF signals received within channel 52 of feeder link 20 to the frequency of the corresponding subband of mobile link channel 63. The satellite transmits the RF signals over mobile link channel 63 in the beam covering the mobile terminal 12. Satellite 10 similarly receives RF signals within a designated subband of a designated channel of the second feeder link 22 from the second earth station 16 and translates same in frequency to a corresponding subband of mobile link channel 64 for retransmission over mobile link 18 to mobile terminal 12.
FIG. 4A illustrates an exemplary mapping scheme correlating a feeder link to multiple beams. A portion of the frequency spectrum of the feeder link is illustrated by reference 80. The feeder link is divided into a plurality of feeder channels 82 numbered #1-#6. FIG. 4A further illustrates frequency spectra for mobile link beams 84-86 corresponding to beams N-1, N and N+1 within a satellites coverage area. Mobile link beams 84-86 are divided into mobile link channels 88-90, respectively. In the example of FIG. 4A, each mobile link beam is divided into four mobile link channels of equal bandwidth. The bandwidth of each feeder link channel 82 equals the bandwidth of each mobile link channel 88-90. The feeder and mobile link channels 82 and 88-90 are occupied by modulated carriers at center frequencies 92 and 94-96, respectively, distributed evenly along the channel.
For purposes of illustration only it may be assumed that feeder link channel #1 is mapped or assigned to mobile link channel #4 within mobile link beam 84, while feeder link channel #6 is assigned to mobile link channel #1 of mobile link beam 86. Feeder link channels #2-#5 are assigned to mobile link channels #1-#4 of mobile link beam 85. According to this example, when an earth station transmits an RF signal at a carrier center frequency 92 within feeder link channel #1, the satellite translates and re-broadcasts the RF signal at a corresponding carrier center frequency 94 within mobile link channel #4 of mobile link 84 supported by beam N-1. Mobile terminals which are located within beam N-1 and are assigned to the broadcast carrier center frequency 94 within mobile link channel #4 of the mobile link 84 receive and process the RF signal. The mobile terminal thereafter transmits RF signals (upon a carrier center frequency assigned thereto by the earth station) within a separate mobile link channel (not shown) of beam N-1. The satellite detects the RF signal transmitted by the mobile terminal and translates the RF signal to a carrier center frequency within a separate feeder link channel (not shown) and transmits same to the earth station.
It should be understood that the mobile uplinks and downlinks do not necessarily share the same band allocations. Nor do feeder uplinks and feeder downlinks share the same bands. For example, the mobile uplinks and downlinks may be transmitted in the L-band and S-band, respectively. For example, the feeder uplinks and downlinks may be transmitted at 30 GHz and 20 GHz, respectively.
In the example of FIG. 4A, the feeder and mobile link channels 82 and 84-86 have equal bandwidths. However, these bandwidths may differ depending upon the frequency spectrum allocated to the feeder link and to the mobile links and depending upon the number of beams generated by the satellite.
FIG. 4B illustrates an alternative feeder link and mobile link channel configuration in which the feeder link channels all have equal bandwidths, while the mobile link channels have different bandwidths. For instance, mobile link channels #2 and #3 in beams #70-72 have the same bandwidth as all feederlink channels #20-25, while mobile link channels #1 and #4 in beams #70-72 all have bandwidths less than the feeder link channels #20-25. In this example, feeder link channels #20, #22, #23 and #25 may be mapped or assigned to mobile link channel #3 of beam 70, channels #2 and #3 of beam 71 and channel #2 of beam 72, respectively. Feeder link channel #21 may be mapped or assigned to mobile link channel #4 in beam 70 as well as mobile link channel #1 in beam 71. Feeder link channel #24 may be mapped or assigned to mobile link channel #1 in beam 72 as well as mobile channel #4 in beam 71. Dashed lines 74 and 75 illustrate overlap regions of the feeder link channels #21 and #24 which may be assigned to two beams.
Optionally, the subbands of feeder link channels #21 and #24 within the overlap regions 74 and 75 may be assigned to only one of the two potential beams at any given time. For instance, the overlap region 74 of feeder link channel #21 may be assigned to either mobile link channel #4 in beam 70 or mobile link channel #1 in beam 72 but not both simultaneously. Similarly, the overlap region 75 of feeder link channel #24 may be assigned to either mobile link channel #1 in beam 72 or mobile link channel #4 in beam 71.
Alternatively, the overlap regions 74 and 75 within the feeder link channels #21 and #24 may be assigned simultaneously to both associated beams provided the earth station maintains synchronization in timing and frequency for RF signals to and from mobile terminals within beams N-1, N and N+1. If the system maintains synchronization in timing and frequency between all mobile terminals, then a mobile terminal in beam N and a mobile terminal in beam N-1 may be assigned to a subband with a common center frequency in the overlap region 74. In this example, interference between these terminals is avoided because each mobile terminal would be assigned a unique CDMA chip code from an orthogonal set. In this alternative configuration, beams N and N-1 would each transmit all RF signals associated with the subbands in the overlap region 74 of channel #21.
FIG. 5 illustrates a block diagram of a channelizer unit which has been proposed by Robert Parish, and the inventors of the present invention. The invention illustrated in FIG. 5 is assigned to the assignee of the present invention.
The forward channelizer unit 100 includes a forward expander 102 and a plurality of forward sub-channelizer units (one of which is illustrated within the block denoted by reference numeral 104). The forward expander 102 may receive up to three forward feeder link signals upon lines 106-108. For instance, lines 106 and 107 may receive the feeder link signals 20 and 22 from first and second earth stations 14 and 16, respectively (FIG. 1). The feeder link signals are processed within signal processing sections 110-112 and passed to a summer 114 which combines the feeder link signals. The composite feeder link signal is passed along line 116 to a power divider 118 which supplies identical copies of the composite feeder link signal to multiple output leads 120. Each of the output leads 120 is connected to a separate forward sub-channelizer unit 104. Each forward sub-channelizer unit 104 is similar in structure and thus only one is illustrated.
Each forward sub-channelizer 104 includes a forward up-converter module 122 and a forward matrix switch module 124. The forward up-converter module 122 converts the composite feeder link signal to a plurality of mobile link signals, each of which corresponds to a mobile link channel. The forward up-converter 122 includes a plurality of power dividers 126 and 128 that divide each incoming composite feeder link signal into a plurality of identical divider output signals, each of which contains all of the RF signals in the composite feeder link signal. The divider output signals are supplied upon leads 130 to an equal plurality of up-converters 132. Optionally, the divider output signals may each pass through filters (not shown) in the up-converters to pass only a desired channel from the composite feeder link signals. Each up-converter 132 is driven by a corresponding unique synthesizer 134. Each synthesizer 134 generates a local oscillator signal which is located at a predefined unique frequency. Optionally, the local oscillator frequency may be selectable from a subset of predefined frequencies, under control of a ground station. Each up-converter 132 translates or "tunes" the frequency of the RF signals from the incoming feeder link intermediate frequency upward by a predetermined amount dependent upon the frequency of the synthesizer 134 to produce a retuned processing channel signal upon lead 136. By way of example only, the feeder link intermediate frequency signals on lines 106-108 may have frequencies in the UHF range, while the processing channel signals output by the up-converters 132 may have frequencies in the L-band or S-band ranges.
The forward matrix switch module 124 includes a plurality of summers 138-141 and 156-159 and eight-way switches 153-155. The summers 138-141 and 156-159 each receive processing channel signals upon at least one dedicated traffic channel (denoted by references 142-149). The first and second eight-way switches 153 and 154 draw inputs from the upper summing group corresponding to summer 126, while eight-way switch 155 receives an input from the lower summing group corresponding to summer 128. The eight-way switches 153-155 perform switching operations to redirect the processing channel signals received upon lines 150-152 to one of the summers. This switching function provides back-up capability in the event of a failure of an upconverter or synthesizer associated with one of the dedicated traffic channels, or alternatively applies an additional increment of feeder link bandwidth to a beam whose traffic demand exceeds the bandwidth capability of the dedicated traffic channel. By way of example only, the first eight-way switch 153 may switch the processing channel signal received upon line 150 to the first summer 138 (corresponding to beam 1). The eight-way switches 153-155 may be controlled remotely by a ground station which instructs the channelizer to switch based upon the external demands placed upon each beam.
For instance, beam #1 may pass over a major metropolitan area and require more resources, while beam #4 may pass over a rural area. Accordingly, the ground station may instruct the channelizer to switch eight-way switches 153-155 to connect each of leads 150-152 to the first summer 138 corresponding to beam #1. The forward sub-channelizer unit 104 provides a "frequency selectable conversion process" which steers or directs the feeder link frequency spectrum to a desired segment of the bandwidth associated with the mobile links.
Optionally, filters may be relocated from the input section of up-converters 132 to the outputs of summers 138. The band pass filters will permit only desired channels of the overall bandwidth to be passed to each corresponding beam. For this approach to offer flexible bandwidth assignments to beams, a plurality of filters of differing bandwidths can be provisioned at the outputs of each of the summers 183, along with a switch network.
However, the forward channelizer in FIG. 5 includes some undesirable characteristics. First, the summing function implemented by summer 114, FIG. 5, has the effect of limiting the total bandwidth of the system to that of a single feeder link, and requiring continuous coordination among the ground stations to resolve conflicts and avoid interference. It would be preferable to preserve the full bandwidth available from the sum of all feeder links and avoid the need for continuous real-time conflict resolution among the ground stations. Secondly, the channelizer 100 requires a separate commandable synthesizer for each up-converter and each processing channel in order to perform retuning, thereby unduly complicating the overall system. Moreover, many such synthesizers must respond to tuning commands, which the ground stations must send either in real-time or in stored schedules for later execution by the payload channelizer to perform retuning during ground station handovers and beam handovers.
Yet another undesirable characteristic of the channelizer of FIG. 5 is that no connection exists between sub-channelizer units 104. Thus, each sub-channelizer unit 104 is only able to map feederlink channels onto a limited number of mobile link channels within an equally limited number of beams. For example, if traffic demand peaks simultaneously in two or more of beams #1-7 serviced by the sub-channelizer 104 shown in FIG. 5, the capacity of channels on leads 142-152 may be exceeded. At the same time, beams other than #1-7 may be under less demand and the corresponding second subchannelizer may have unused channels. However, those unused channels cannot relieve a shortage in beams #1-7 because there is no connection between sub-channelizer units. Additionally, the channelizer 100 requires that the bandwidth requirements of each beam be predicted in order that the ground station may set the eight-way switches 153-155 accordingly. However, bandwidth requirements continuously change as satellites orbit the earth. As satellites orbit, the coverage area associated with a given satellite continuously moves and thus will pass over metropolitan and rural areas having different resource demands. It is difficult to predict when resource demands will change. Imperfect predictions lead to wasted resources and lost revenue for the system operator.
Moreover, as the satellites orbit, fixed and mobile terminals pass between beams. When a terminal passes between beams, a beam hand over must take place to maintain the terminal's connection with the system. The handover process requires coordination between ground stations, terminals and satellites to avoid dropping a user. To effect a handover, the switching configuration within the forward matrix switch module 124 must be changed and the synthesizer frequency selection must be retuned. Switching and retuning unduly complicate the hand over process. When in high demand areas, switching and retuning unduly increase the risk of dropping users while a resource is switched between beams. The risk of being dropped may exist each time a satellite effects a handover while passing over a given area. Satellites follow the same ground track multiple times per day and thus, mobile terminals in the same regions on the earth will undergo handovers at the same time each day. Thus, a user located within an area associated with a high risk of dropouts may experience the same problem at the same time each day.
A need remains for an improved channelizer for use with a satellite based telecommunications system. It is an object of the present invention to meet this need.